I have a 5.0 EFI form a 87' Mustang. I know originally both heater hoses (inlet/outlet) were 5/8".

When I bought my car the inlet and outlet used are on the back of the intake manifold. The upper hose is 3/4" and the lower is 5/8". The guy bypassed the heater core by using a 3/4" to 5/8" adapter.

Obviously the 3/4" inch is too large to fit through the firewall (I could always use a step drill to open up).
The upper heater core nipple is 5/8" so the 3/4" hose would be too large (I could always modify at a radiator shop).

I prefer to use a different nipple or replace the one on the back of the motor. Any ideas on how to convert the 3/4" nipple fitting to a 5/8" fitting?

 

1. I bet the heater core leaks (that is why it was bypassed).... Have the rad shop test and inspect it.
2. Perhaps you could use the adapter with the two sizes of hose where it will be unseen... (best/easisest solution)
3. If the nipples are screwed in - i assume they are not - or you would not ask... then swap for two lower (smaller) ones with a pipe fitting reducer.